1. Field of the Invention
The present invention relates to a control system for an engine mounted on a vehicle, and, more particularly, to engine control system for reducing vibrations of a driveline including an engine.
2. Description of Related Art
Motor vehicles often cause discomfort vibrations from a driveline during acceleration that imposes an increasing load on an engine. Such a discomfort vibration occurs resulting mainly from torsional vibration in a driveline, in particular a drive shaft, of the vehicle when engine dynamic torque, that increases as an engine load increases, contains a torque component coincident in frequency with a driveline resonant torsional vibration which is hereafter referred to as a resonant frequency torque component. The driveline resonant torsional frequency typically depends upon a gear ratio of a transmission of the driveline. It has been proposed with an attempt to reduce discomfort vibrations occurring in the driveline in, for example, Japanese Unexamined Patent Publication No. 8-232696 to reduce engine torque by reducing an amount of fuel supply to the engine in a specific pattern of acceleration where the engine dynamic torque results in containing a resonant frequency torque component depending upon a gear ratio of a transmission of the driveline, in other words, in a specific pattern of engine operation where an engine dynamic load increases in a specific pattern. It has also been proposed in that publication to cause the reduction of fuel supply with a delay by approximately one revolution of engine from a point of time at which an occurrence of the specific pattern of acceleration is detected.
In the prior art engine control system disclosed in the publication, there are established several typical patterns of acceleration where engine dynamic torque results in containing a resonant frequency torque component. The engine control system executes the torque reduction control for the purpose of reducing driveline torsional vibrations on condition that it is detected that the engine has entered one of the patterns of acceleration.
However, there are conceivably innumerable patterns of acceleration where engine dynamic torque results in containing a resonant frequency torque component. When taking the fact that the driveline resonant torsional frequency varies depending upon a transmission gear ratio, in addition to the innumerable patterns of acceleration, into account, there is still room for improvement in sufficiently reducing driveline torsional vibrations. That is, even when the engine is operating in a certain pattern of acceleration in which the engine dynamic torque contains a resonant frequency torque component, the control of engine torque reduction is not achieved unless the pattern of acceleration is one of the specific patterns of acceleration as having been established. Accordingly, there are a great number of chances for the driveline to produce torsional vibrations.
It is an object of the present invention to provide an engine control system for reliably reducing driveline torsional vibrations during acceleration.
The above object of the present invention is accomplished by an engine control system for controlling an internal combustion engine mounted on a vehicle that is operative basically to predict, at the beginning of acceleration, a change in a value relating to an engine load that is expected to be imposed on an engine during the acceleration with time, to extract a component of the change coincident in frequency with a driveline resonant torsional vibration (which is referred to as a resonant frequency component) and to reduce engine torque by an amount meeting to the resonant frequency component.
Specifically, the engine control system comprises throttle opening detection means for detecting a throttle opening of the engine, rotational speed detection means for detecting a rotational speed of the engine, and control means that includes prediction means for predicting an engine load and a change in the engine load with time in a specified period of time from an occurrence of driver""s operation for causing an increase in throttle opening on the basis of the throttle opening and the rotational speed when the occurrence of driver""s operation is detected on the basis of a result of detection by the throttle opening detection means, filter means for extracting a resonant frequency component that is coincide with a resonant frequency of a driveline of the vehicle from of the change in engine load predicted by the prediction means, and control means for performing torque reduction control which diminishes engine torque by an amount meeting the resonant frequency component at a specified torque reduction timing corresponding to an occurrence of the resonant frequency component.
The throttle opening may be detected in the form of, for example, an accelerator position or travel. The rotational speed of engine may be detected in the form of, for example, a vehicle speed or a rotational speed of turbine of a torque converter. Further, the engine load may be detected in the form of a charging efficiency. The specified torque reduction timing may be a point of time after a lapse of specified time from stepping on an accelerator pedal.
According to an embodiment of the engine control system, because engine torque is reduced by an amount meeting a resonant frequency component of a change in engine load with time during acceleration that is predicted at the beginning of the acceleration, while acceleration during which driveline torsional vibrations occur due to the resonant frequency component can be reliably detected, the driveline torsional vibration due to the resonant frequency component is reliably diminished. Although it is conceivably possible to reduce engine torque when detecting vehicle body vibrations through feedback control, such feedback control includes a problem of responsiveness because it is too late to reduce engine torque following detection of vehicle body vibrations and is, as a matter of fact, hardly employable for engine control systems.
In the engine control system, the specified torque reduction timing may be changed according to the rotational speed of engine so as thereby to cause a reduction in engine torque earlier. More specifically, the specified torque reduction timing is changed by a degree of advance that is made greater with a drop in the rotational speed of engine. This prevents reduction of engine torque from being performed with a time lag due to a delay in calculation when the rotational speed of engine is comparatively low and, as a result, reliably diminishes the driveline torsional vibration due to the resonant frequency component.
The control means may interrupt the torque reduction control when an engine load is lower than a specified value. This is more desirable in terms of simplifying the torque reduction control and satisfying an acceleration demand because driveline torsional vibrations are practically insignificant while an engine load is low from the outset.
In the case where a continuously variable transmission is installed in the driveline, the control means diminishes a reduction in engine torque by an amount meeting inertia in the continuously variable transmission when the continuously variable transmission varies a gear ratio thereof This is because, while the continuously variable transmission is generally controlled to shift a gear toward a lower gear during acceleration, the continuously variable transmission dissipates engine torque during the gear shift. The demanded amount of engine torque reduction is more optimized for diminishing driveline torsional vibrations by diminishing the engine torque reduction by an amount meeting the dissipation of engine torque during the gear shift.
According to another embodiment of the engine control system, the control means performs the torque reduction control by retarding an ignition timing when the demanded amount of engine torque reduction is smaller than a first specified amount, performs the torque reduction control by cutting fuel injection when the demanded amount of engine torque reduction is larger than a second specified amount that is larger than the first specified amount, and performs the torque reduction control at the specified torque reduction timing by only retarding the ignition timing and further causes an increases in engine torque at a timing later than the specified torque reduction timing so as to diminish an engine torque fall occurring immediately after an engine torque rise occurring due to the resonant frequency component when the demanded amount of engine torque reduction is between the first and the second specified amount.
The engine control system thus structured achieves the reduction of engine torque with high precision by retarding the ignition timing when the demanded amount of engine torque reduction is smaller than the first specified amount and reduces engine torque sufficiently by cutting fuel injection when the demanded amount of engine torque reduction is larger than the second specified amount. Further, the engine control system prevents or sufficiently diminishes accelerative torsional vibrations by increasing engine torque so as to make up a torque fall out of fluctuations of the resonant frequency component when the demanded amount of engine torque reduction is between the first and the second specified amount.
In this instance, when the demanded amount of engine torque reduction is between the first and the second specified amount, the control means makes a first torque rise occurring at the beginning of acceleration as small as possible by changing the ignition timing by the greatest degree of retardation and, in consequence, significantly diminishing a torque fall immediately after the first torque rise. This diminishes accelerative vibrations effectively on the whole. Furthermore, the engine control system causes an increases in engine torque meeting a difference of a demanded amount of engine torque reduction resulting from changing the ignition timing by the greatest degree of retardation from the demanded amount of engine torque reduction meeting the resonant frequency component. This makes it easier to set an amount of engine torque increase. The increase of engine torque may be achieved by either making an air-fuel ratio leaner or increasing an amount of intake air. This is desirable to realize the engine torque increase simply and precisely.
According to still another embodiment of the engine control system, the control means achieves the demanded amount of engine torque reduction meeting the resonant frequency component by making an air-fuel ratio leaner when an air-fuel ratio after having been made as lean as necessary to cause the demanded amount of engine torque reduction is out of a specific range of air-fuel ratio for a level of NOx emission in an exhaust gas higher than a specified level and, on the other hand and by retarding an ignition timing when the air-fuel ratio after having been made as lean as necessary to cause the demanded amount of engine torque reduction is within the specific range of air-fuel ratio.
More specifically, the engine control system performs the torque reduction control with a specific air-fuel ratio on a leaner side from the specific range of air-fuel ratio when the air-fuel ratio after having been made as lean as necessary to cause the demanded amount of engine torque reduction is leaner than the specified air-fuel ratio and further performs the torque reduction control by retarding an ignition timing so as to make up a shortage of engine torque reduction when the torque reduction control with the specific air-fuel ratio causes the shortage of engine torque reduction relative to the demanded amount of engine torque reduction. Further, the engine control system performs the torque reduction control by cutting fuel injection when a total amount of engine torque reduction of both amounts of engine torque reduction resulting from the torque reduction control by using the specific air-fuel ratio and the torque reduction control by retarding an ignition timing is smaller than the demanded amount of engine torque reduction.
The engine control system thus structured performs the reduction of engine torque for diminishing accelerative vibrations by making utilization of a lean air-fuel ratio that is advantageous to improvement of fuel efficiency and, on the other hand, prevents or significantly suppresses an increase in NOx emission by making utilization of a retardation of ignition timing.